Horticulture

Leaf infected with Myrtle rust

Photos: Brett Summerell and John Siemon, © Royal Botanic Gardens and Domain Trust PLANT BREEDING IS A COMPLEX PROCESS AND SOMETIMES THE STAKES ARE EXCEPTIONALLY HIGH, WITH SCIENTISTS RACING TO STOP PATHOGENS WIPING OUT ENTIRE SPECIES. JOHN SIEMON REPORTS.

It’s easy to take for granted the importance of genetic diversity. Few shoppers, when purchasing a bunch of bananas, a bag of potatoes or a loaf of sourdough, pause to consider the chain of custodians that have brought the product to their fingertips. Pushed to list those involved, they might mention the farmer, the truck driver or the baker. But would they think about the agronomist, the plant pathologist or the plant breeder, who were equally, if not more important in ensuring the careful selection and cultivation of genetically diverse cereal grains that were resilient to an army of evolving plant pathogens?

Many of our decisions involving plants, whether we like it or not, have been determined for us, and in many cases that process took place years before it touched our lives. In the case of plant breeding, the timber frames that make up modern house construction may have arisen from forestry tree plantation breeding programs 30 years ago. And think about the produce that we buy from supermarkets or fruit shops – its domestication may have started thousands of years ago, with active breeding programs in more recent years improving yield, disease resistance, flavour, vigour or shelf life.

In the ornamental horticulture world, many advances in plant breeding have occurred by exploiting natural variation with minimal breeding. Sometimes exceptional forms of plants arise by chance or because they have an evolutionary advantage. For example, floriferous specimens are more likely to be visited by pollinators and therefore more capable of producing seeds, improving the chances of survival for more individuals in the next generation. Plant breeders with a keen eye can identify attributes most likely to be sexy in a plant pot, and cross pollinate individuals of successive generations until the desired attributes are achieved.

Extensive plant breeding over millennia has enabled global human populations to explode, and today feeding an ever-increasing number of mouths with diminishing resources is a

key focus of agencies across the globe. Scientific research into plant groups such as cereals (wheat, barley, oats, rice, maize and rye) is a serious business. The Poaceae or grass family is attacked by a plethora of plant pathogens, often rust diseases, that aggressively target the stem and leaves. The rust spores are so light they are known to drift between continents. In the ultimate game of chess, plant breeders must ‘insert’ a suite of rust resistant genes to ensure farmers successfully stay ahead of the wave of diseases and minimise the use of chemicals. The seasonal human influenza vaccine, which is amended by scientists annually to incorporate the latest flu strains circulating the globe, is a comparable process, except breeding a single wheat cultivar to achieve the same resistance can take five to seven years to reach production scale.

Meanwhile, botanic gardens and plant conservation agencies are confronted with an even more challenging feat: slowing or halting the total global extinction of species under attack from a barrage of threats.

In the case of biological threats such as Myrtle rust (Austropuccinia psidii), which targets trees and shrubs in the Myrtaceae family (including eucalypts, lemon myrtles and tea-trees), we have seen in Australia a rapid decline in species that are vulnerable to the disease. Currently, teams at the Australian Institute of Botanical Science, in partnership with other government agencies, are racing against time to collect sufficient wild germplasm of three highly-sensitive species – Rhodamnia rubescens, Rhodomyrtus psidioides and Lenwebbia sp. Main Range (P.R.Sharpe+ 4877) – before the disease wipes out every last specimen (see separate story).

Many of our conservation programs aspire to ensure we hold representative genetic diversity of a species. The Australian PlantBank does this very effectively by storing millions of seeds from more than 5,000 species. However, when a species is under immense pressure for survival, seed production can fall or halt completely, hampering efforts to conserve these plants through seed methodologies and long-term storage.

For some species, an ex-situ conservation program requires cultivation in ground or in potted nursery situations. Unlike the Wollemi Pine, which can be readily propagated, grown and planted in many environments around the globe, species sensitive to Myrtle rust cannot readily go back into an environment where the disease is present without significant horticultural management.

No plant should ever be functionally extinct (i.e. no longer present naturally in the wild), but that’s the reality we are faced with because of biodiversity threats like Myrtle rust. Short of finding global locations that are disease-free, we can only deploy our expertise to buy plants time and explore how scientific techniques, such as plant breeding and emerging technologies, might introduce natural rust-resistant genes that effectively help ‘vaccinate’ plants before reintroducing them back into

Propagating Lenwebbia sp. Main Range (P.R.Sharpe+ 4877)

‘Rust spores are so light they are known to drift between continents’

RACING AGAINST RUST In the fight against Myrtle rust, one of the challenges facing scientists is that critically endangered species, such as Lenwebbia sp. ‘Main Range’, show little variation in levels of disease resistance. In other words, instead of displaying symptoms ranging from mild to extreme, infected plants simply experience repeated defoliation events and eventually die (along with all their seedlings). This makes it difficult to select and breed individuals with rust resistance, so scientists are exploring alternative solutions such as introducing resistance from other related species through complex plant breeding and genetic engineering. At the same time, Myrtle rust is now so widespread along Australia’s eastern seaboard that there are few, if any, geographically and climatically suitable locations that are uninfected. Relocating susceptible species to other parts of Australia is not currently an option as it could result in the pathogen spreading even further afield. The Gardens recently explored Myrtle rust in episodes of its Branch Out podcast series rbgsyd.nsw.gov.au/Science/BranchOut and its What the Flora?! video series rbgsyd.nsw.gov.au/Science/ What-the-Flora-!.